Flexible magnetic recording media generally comprises magnetic pigment or particles, polymeric binder, lubricant, dispersant, and other minor additives. The majority of magnetic particles of practical importance are metal oxides. Interactions which exist between the magnetic particles and the binder can effect the frictional characteristics of the media, such as tape.
Particle-binder interactions that are desirable from the standpoint of tape-media performance are those interactions which maintain separation of the individual particles, reinforce the mechanical properties of the binder, and hold the particles to the tape's flexible substrate in a cohesive coating. Undesirable interactions between the binder and the magnetic particles can lead to deterioration of magnetic performance or to deterioration of the coating's mechanical properties.
Interaction between the binder and the particles is aggravated significantly by the fact that the majority of magnetic coatings contain magnetic-oxide particles in excess of 70% of the coating by weight and as much as 50% by volume. In order to achieve these high particle loadings, strong interactions between the particle and the polymeric binder are necessary.
Polyester-polyurethanes (a type of thermoplastic elastomer) are widely used as binders for flexible magnetic recording media. These binders are rubbery materials which can be melted and cooled reversibly, without major changes occurring in their chemical or physical properties. Their unique properties, which are a direct result of the block-copolymer nature of these materials, make them significantly different from other elastomers, such as natural or synthetic rubber.
These materials are composed of segments or blocks of chemically different units. The polyester portion, or soft segment, is composed of a repeating series of ester-linked units, and is itself a short-chain-length polymer. The polyester segments are formed by the reaction of a difunctional carboxylic acid with a difunctional alcohol, such that the ester is terminated substantially with alcohol or hydroxyl end groups. This polyester portion typically has a molecular weight of from 500 to 4000, corresponding to chains composed of from 4 or 5 ester units to as many as 20. The effect of an increase in the length of the soft segment is generally an increase in the elasticity of the polyurethane. In general, it is the soft-segment portion of the polyester-polyurethane that determines the low temperature and the elastomeric properties of the binder.
The other component in the polyester-polyurethane is the polyurethane or hard segment portion. This portion possesses a markedly different chemical and mechanical behavior from that exhibited by the polyester soft segments. In general, the hard segment is a hard, rigid polymer with a melting point near 200.degree. C. The hard segment is usually produced from a difunctional, aromatic diisocyanate, such as 4,4'-diphenylmethane diisocyanate (MDI) which is reacted with a difunctional alcohol such as 1,4-butanediol. The hard segment usually has a very short chain length in the case of polyester-polyurethane elastomers used in solvent-based magnetic media coatings because the hard segment is not particularly soluble in the common solvents (examples are THF and MIBK) used in the manufacture of magnetic tape coatings. In addition, the size of the hard segment blocks has been found to increase hardness, modulus, and flow temperature at the expense of elasticity and toughness. For flexible magnetic recording tape, a balance of properties is sought so that the binder can be adapted to the requirements of magnetic recording.
It has been found, in accordance with the suggestions in U.S. Pat. No. 4,284,750, that thermoplastic polyurethane compositions having excellent mechanical and thermal properties, high hardness, and the capacity of binding or adhering to magnetic pigments can be formed by reacting (A) cyclohexanedimethanol and an acid selected from the group adipic acid, azelaic acid, and 1,12-dodecanedioic acid, including mixtures thereof; (B) a chain extender such as 1,4-butanediol; and (C) a diisocyanate such as MDI (methylene bis diphenyl diisocyanate, also known as 4,4'-diphenylmethane diisocyanate).
It has been found, however, that when magnetic chromium dioxide is employed as the ferromagnetic pigment, in place of iron oxide particles with binders disclosed in U.S. Pat. No. 4,284,750, certain problems are encountered. For instance, if one follows the teachings of U.S. Pat. No. 4,284,750 when making chromium dioxide magnetic recording media, a substantial decay in the media's mechanical properties, such as the modulus (i.e., hardness, stiffness, load bearing capacity), occurs within the temperature range of 10.degree. C. to 50.degree. C. If one tries to improve the media by thermal annealing, only slight improvement results.
In U.S. Ser. No. 567,291, certain problems concerning coating performance are overcome. In particular, it has been found, according to said application, that polyester-polyurethanes, of the general type defined in U.S. Pat. No. 4,284,750, become satisfactory for use with chromium dioxide particles when the polyurethane possesses increased hard segment content in the range 37% to 40% by weight and, preferably, 40% with the soft segment molecular weight being in the range of about 500 to 1500.
However, it has been found that such binders are not entirely satisfactory from a processability standpoint. In particular, such binders tend to be brittle. Also, such binders have a tendency to lose cohesive integrity and adhesion to substrates such as polyethylene terephathalate substrates.
Further, concerning various contributions recognized by the prior art to the structural and mechanical properties of polyurethanes made by the hard segment content, attention is directed to the following publications.
R. J. Zdrahala, et al., "J. Elast. Plast.", Vol. 12, p. 184, 1980.
S. L. Cooper and A. V. Tobolsky, "J. Appl. Poly. Sci.", Vol. 10, p. 1837, 1966.
K. C. Frischland and S. L. Reegen, Ed., "Advances in Urethane Sci. Tech.", Vol. 3, pp. 36-65, 1974.
T. E. Lipatova, et al., "Poly. Sci. U.S.S.R.", Vol. 20, p. 2305, 1979.
W. Nierzwicki and E. Szpilewicz, "J. Appl. Poly. Sci.", Vol. 23, p. 2147, 1979.
R. J. Zdrahala, et al., "J. Elast. Plast.", Vol. 12, p. 225, 1980.
C. S. Schollenberger, "Advances in Chemistry Series 176", American Chemical Society, 1979.